The Relationship of Heat Killing and Thermal Radiosensitization to the Duration of Heating at 42°C

Thermal cell killing and/or thermal radiosensitization were evaluated in asynchronous Chinese hamster ovary (CHO) cells subjected to 2 to 12 hr of heating at 42°C. Initially, survival decreased (from 0.30 to 0.10) as the heating interval was increased from 2 to 4 hr. Thermotolerance then developed after 4 to 6 hr of heating; i.e., after 6 hr of heating, survival did not decrease below 0.06. Similarly, when the cells were given graded doses of X radiation, after being heated for the various durations, the amount of thermal radiosensitization did not increase beyond the degree experienced for 2 hr of heat; i.e., the D0 decreased from 150 rad for radiation alone to 100 rad for heat (2 to 12 hr) plus radiation. This decrease in D0 may reflect a population of G1 cells which survived killing by the heat treatment and cannot be interpreted as thermal radiosensitization per se. Treatment with the combined modalities did not change the extrapolation number (n) compared to radiation alone.